This work is an investigation of post-transcriptional gene silencing (PTGS) in plants, a process that mediates sequence-specific degradation of RNA. Initially discovered in transgenic plants, PTGS has been long regarded as a curiosity, or even as an artefact of transgenesis. It is shown here that virus-induced gene silencing, in which recombinant viruses carrying element of the host genome trigger PTGS of the corresponding plant gene (Chapter one), is a manifestation of a defence system. This defence is remarkable in its ability to adapt to potentially any virus because its specificity is not genetically programmed by the host but, instead, is dictated by the genome sequence of the viral intruder itself. It is demonstrated in chapters 4 and 5 that PTGS of a transgene can spread in plants from one part to another, indicating the existence of a systemic, sequences-specific silencing signal that is likely to have a nucleic acid component. From the demonstration that replication of potato-virus X also triggers production of a silencing signal in non-transgenic plants (Chapter 8), it is proposed that this long-distance signalling process represents the systemic arm of the host PTGS defence response. Collectively, these findings define the existence of a previously uncharacterised antiviral mechanism in higher plants, which may also operate in animals. This defence holds key features of an elaborate immune system, as it is adaptive, mobile and specific. It is also shown, here, that plant viruses have elaborated counter-defensive measures to overcome the host PTGS response, by producing suppressor proteins that target various steps of the silencing mechanism (Chapters 6, 7). One of these factors, the PYX-encoded p25 protein, had been previously characterised as a facilitator of viral cell-to-cell movement. The finding that p25 specifically inhibits the signalling step of PTGS (Chapter 8) provides a new ground for the investigation of virus movement in plants. In chapter 9, the role of PTGS in plants and its suppression by viruses is discussed in the broader context of plant development and biotechnological applications.